In recent years, in response to strong social demands for low fuel consumption of vehicles, weight reduction for tires has been sought in every tire members. Weight reduction of an inner liner has also been sought. The inner liner is disposed on the inner side of a tire, and serves to improve air permeability resistance by reducing an amount of leakage of air (amount of permeating air) from inside to outside of the pneumatic tire.
Currently, a rubber composition for such an inner liner employs, for example, a rubber blend mainly containing a butyl rubber. The rubber blend contains the butyl rubber by 70 percent by mass to 100 percent by mass, and a natural rubber by 30 percent by mass to 0 percent by mass. In this way, the tire is provided with improved air permeability resistance. In addition to butylene, the rubber blend mainly containing the butyl rubber contains isoprene by approximately 1 percent by mass, which acts with sulfur, vulcanization accelerator, and zinc white to achieve co-crosslinking between molecules with an adjacent rubber layer. In the case of normal blend, the above-described butyl-based rubber needs to have a thickness of 0.6 mm to 1.0 mm for a tire of a passenger car, and needs to have a thickness of approximately 1.0 mm to 2.0 mm for a tire of a truck/bus. In order to achieve weight reduction of such tires, polymer which is more excellent in air permeability resistance and can provide an inner liner layer with a thinner thickness than the butyl-based rubber has been requested.
There is a conventional technique of using a thermoplastic elastomer for weight reduction of the inner liner layer. With this technique, however, if the thickness is made thinner than an inner liner of butyl rubber, it is difficult to achieve air permeability resistance and weight reduction at the same time. Moreover, making the thinness smaller decreases the strength of the inner liner, with the result that the inner liner may be broken due to heat and pressure of a bladder during a vulcanization step. Furthermore, in the case of thermoplastic elastomer of low strength, cracks are likely to be produced in the inner liner at a buttress portion which is subjected to large repeated shearing deformation during traveling with the tire.
Conventionally, in order to achieve weight reduction of a tire, it has been proposed to use a film made of a material containing a thermoplastic resin, instead of the above-described rubber composition. However, the tire is left outdoor during transportation or display at a dealer, and suffers from degradation due to ultraviolet radiation of sunlight, so that a thermoplastic elastomer deteriorates to cause a crack, resulting in an impression of bad inner appearance. Moreover, since a pneumatic tire is filled with air in its inner space during use, oxygen in the air will permeate the inside of components constituting the tire, causing oxidation to progress with time. An adverse influence will thus be effected on durability of the pneumatic tire. In particular, if a crack occurs in the inner liner, an impression of bad inner appearance is given to a user. Furthermore, gas barrier property partially deteriorates to decrease tire internal pressure.
In addition, during traveling with the tire, large shear strain acts on the vicinity of a shoulder portion in the inner liner. When the material containing a thermoplastic resin is used as the inner liner, this shear strain is likely to cause detachment at an adhesion interface between the inner liner and the carcass ply, with the result that air leakage takes place from the tire, disadvantageously.
Meanwhile, in order to achieve weight reduction of the inner liner, a technique of employing a thermoplastic elastomer material also has been proposed. However, it is known that a material, which is made thinner in thickness than the inner liner of butyl rubber and exhibits high air permeability resistance, is inferior to the inner liner of butyl rubber in view of vulcanization adhesion strength with an insulation rubber or a carcass ply rubber adjacent to the inner liner.
When the inner liner has low vulcanization adhesion strength, air enters between the inner liner and the insulation rubber or the carcass rubber, thus resulting in a so-called air-in phenomenon, in which small balloon-like objects appear. The multiplicity of such small spots in the tire gives a user an impression of bad appearance. In addition, during traveling, the air causes detachment to result in cracks in the inner liner. Accordingly, the tire internal pressure is decreased.
Japanese Patent Laying-Open No. 9-019987 (PTD 1) discloses a layer stack for improving adhesive property between an inner liner layer and a rubber layer. By providing adhesion layers on the opposite sides of the inner liner layer, the adhesion layers come into contact with each other at an overlapping portion of the inner liner layer and are bonded firmly by heating. Air pressure retainability is thus improved. However, the adhesive layers for the overlapping of the inner liner layer will come into contact with the bladder in a heated state during a vulcanization step to stick and adhere to the bladder, disadvantageously.
In Japanese Patent No. 2999188 (Japanese Patent Laying-Open No. 2000-159936 (PTD 2)), a mixture of a nylon resin having excellent air permeability resistance and a butyl rubber is produced by dynamic cross-linking, thereby fabricating an inner liner layer having a thickness of 100 μm. However, the nylon resin is hard at a room temperature, and is therefore not suitable for an inner liner for a tire. Further, the mixture produced by dynamic cross-linking is not enough to achieve vulcanization adhesion with a rubber layer. Hence, an adhesive layer for vulcanization is required apart from the inner liner layer. This results in a complicated structure as an inner liner member as well as increased number of steps, which is disadvantageous in view of productivity.
In Japanese Patent Laying-Open No. 2008-024219 (PTD 3), a flexible gas barrier layer is fabricated by dispersing a maleic anhydride modified hydrogenated styrene-ethylene-butadiene-styrene block copolymer in an ethylene-vinyl alcohol copolymer having excellent air permeability resistance. The layer is sandwiched by thermoplastic polyurethane layers to form a sandwich structure, and a rubber cement (obtained by dissolving a butyl rubber and a natural rubber at 70/30 in toluene) is applied to a surface to be adhered to the tire rubber, thereby fabricating an inner liner layer. However, the modified ethylene-vinyl alcohol copolymer having the flexible resin dispersed therein has low adhesive strength, and may be detached from the thermoplastic polyurethane layers. Moreover, although the modified ethylene-vinyl alcohol copolymer having flexible resin dispersed therein has flexible resin dispersed therein, the EVOH of the matrix has poor flection fatigue resistance, thus resulting in breakage during traveling with the tire. Since the rubber cement is applied to the surface to be adhered to the tire rubber, a step different from the normal inner liner step is required, resulting in degraded productivity.
Japanese Patent Laying-Open No. 2008-174037 (PTD 4) proposes a pneumatic tire having, on the inner side of a carcass layer, an air permeation preventing layer of a thermoplastic elastomer composition containing a thermoplastic resin or a thermoplastic resin and an elastomer, in which an average thickness Gs of the air permeation prevention layer extending from the vicinity of a maximum width end of a belt layer to a tire maximum width region Ts is made thinner than an average thickness Gf of the air permeation preventing layer in a region Tf extending from the tire maximum width to a bead toe, thereby improving flection durability. Such a configuration, however, may cause detachment between a rubber layer of a carcass ply and the air permeation preventing layer.
To simultaneously achieve suppression of air pressure drop, improvement in durability and improvement in fuel consumption, Japanese Patent Laying-Open No. 2007-291256 (PTD 5) discloses a pneumatic tire formed using a rubber composition for an inner liner containing ethylene-vinyl alcohol copolymer by 15 parts by mass to 30 parts by mass with respect to 100 parts by mass of a rubber component made of a natural rubber and/or a synthetic rubber.
In Japanese Patent Laying-Open No. 2009-298986 (PTD 6), titanium oxide is blended with a blend of a butyl-based rubber and a nylon resin for preventing ultraviolet degradation. However, besides ultraviolet degradation, durability is disadvantageously decreased by degradation of nylon resin due to occurrence of radical caused by flection fatigue.
In WO2007/116983 (PTD 7), a light sealing layer in which carbon black is blended with a mold lubricant for preventing ultraviolet degradation of a thermoplastic elastomer layer is provided on a surface layer. However, due to fluctuations in the step of applying the mold lubricant, the mold lubricant cannot be applied uniformly in the inner surface of the tire. If a scratch is made by the hand of an operator in the step or a user, or for another reason, the light sealing layer does not achieve its function, resulting in degraded durability due to ultraviolet degradation.
Japanese Patent Laying-Open No. 2005-343379 (PTD 8) achieves improvement in low temperature durability by designing the thickness at a shoulder part larger than the thickness at a tire crown portion. The thickness at the shoulder part is designed to be thicker than that at the tire crown portion to suppress flection deformation and reduce the occurrence of cracks, however, it is disadvantageous in view of weight reduction of a tire.